General photosensitive resin compositions for flexography commonly comprise an unhydrogenated thermoplastic elastomer, a photopolymerizable unsaturated monomer, and a photopolymerization initiator, as described in Patent Documents 1 to 3.
Raw plates for flexography comprise a support such as a polyester film and the photosensitive resin composition laminated thereon. Further, a slipping layer or protective layer is provided on the photosensitive resin composition, if necessary, for the purpose of achieving smooth contact with a negative film. Alternatively, an ultraviolet blocking layer containing an infrared-sensitive substance which can be cut off by an infrared laser is sometimes provided on the photosensitive resin composition.
A method for preparing a flexographic printing plate from such a raw plate for flexography involves initially subjecting the whole surface of the raw plate to ultraviolet exposure through a support (back exposure) to provide a thin uniform cured layer thereon; subsequently subjecting the surface of a photosensitive resin layer to relief exposure through a negative film or through an ultraviolet blocking layer cut off by an infrared laser; and finally washing out unexposed parts with a solvent for development or removing unexposed parts by the absorption of an absorbing layer after heating and melting, and conducing post-exposure.
Printing using the flexographic printing plate prepared with the photosensitive resin is carried out by supplying inks containing an ester solvent or the like onto the surfaces of convex parts of the resin plate having concaves and convexes by use of an ink-supplying roll or the like, and next bringing the printing plate into contact with a substrate to be printed on to transfer the inks on the surfaces of the convex parts to the substrate to be printed on.
The flexography described above presents such problems that: a printing plate capable of reproducing exact shape onto a negative film (fine line reproducibility) is not obtained; cracks occur on plate surface during printing due to a large post-exposure light amount; a printing plate swells and chips (chipping resistance) during long-duration printing or its washing and parts other than original patterns are also printed due to lack of ester solvent resistance. Various solutions have been proposed to such problems of the photosensitive resin composition.
In Example 1 of Patent Document 3, a styrene-isoprene block copolymer is used as a polymer in a photosensitive resin. A photosensitive resin using this polymer has relatively good resistance to ester solvents. However, the photosensitive resin contains isoprene and therefore, does not necessarily have sufficient prevention of cracks occurring on plate surface and abrasion resistance.
In Patent Document 4, a terpolymer having randomly distributed ethylene, propylene, and diene units is used as a polymer in a photosensitive resin. However, a photosensitive resin plate using this polymer does not necessarily have sufficient fine line reproducibility. Patent Documents 5 to 7 propose a block copolymer having alkylene units as a polymer in a photosensitive resin. In Examples therein, a block copolymer having a styrene block, an alkylene block, and an isoprene block is illustrated. However, all of these photosensitive resins comprise the alkylene block not less than 99 wt % with respect to the total amount of butadiene units and alkylene units and therefore, do not necessarily have sufficient fine line reproducibility, chipping resistance, and prevention of cracks occurring on plate surface.
Patent Document 1: JP-A-2000-155418
Patent Document 2: JP-A-02-108632 (corresponding U.S. Pat. No. 4,894,315)
Patent Document 3: JP-A-04-342258 (corresponding U.S. Pat. No. 5,135,837)
Patent Document 4: European Patent No. 0326977 (corresponding U.S. Pat. No. 4,994,344)
Patent Document 5: JP-A-10-288838 (corresponding U.S. Pat. No. 6,037,101)
Patent Document 6: JP-A-10-73921
Patent Document 7: U.S. Patent No. 20050239930